n-api: wrap control flow macro in do/while

Make CHECK_ENV() safe to use in the following context:

    if (condition)
      CHECK_ENV(env);
    else
      something_else();

PR-URL: https://github.com/nodejs/node/pull/18532


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>